The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device for use in a 1.3 μm or 1.5 μm wavelength band.
Optical wavelength band of 1.3 μm or 1.5 μm is used commonly in optical telecommunication systems that use optical fibers. It should be noted that a quartz glass optical fiber has an optical transmission band in the wavelength of 1.3 μm or 1.5 μm.
In correspondence to the foregoing specific optical transmission band of the optical fibers, conventional optical telecommunication systems generally use a laser diode constructed on an InP substrate. Such a laser diode typically uses an active layer of InGaPAs having a lattice constant matching the lattice constant of the InP substrate and a bandgap corresponding to the optical wavelength of 1.3 μm or 1.5 μm.
While the foregoing laser diode that uses InGaAsP active layer performs well in conventional optical telecommunication systems, particularly optical telecommunication trunks, the laser diode, requiring an expensive temperature regulation system such as a Peltier cooling device for a proper operation thereof, is deemed to be inappropriate for optical subscriber systems such as optical home terminals because of the increased cost of the temperature regulation system. In the foregoing laser diode that uses the InGaPAs active layer in combination with the InP substrate, the discontinuity of conduction band at the interface between the active layer and the surrounding cladding layer or optical waveguide layer is not sufficient for effective confinement of the carriers in the active layer, and there is a tendency that the carriers escape or overflow from the active layer when the device is not properly cooled. Because of such a poor confinement of the carriers, the laser diode generally shows a poor efficiency of laser oscillation. This problem becomes particularly serious in a high temperature operation of the laser diode where the carriers experience extensive thermal excitation.
On the other hand, recent investigations on a GaAs—GaN system have discovered that the bandgap of a GaAs mixed crystal containing therein a small amount of N decreases with increasing N content in the GaAs mixed crystal. GaN itself has been known to have a very large bandgap and is used for an active layer of an LED or laser diode that emits a blue or violet optical radiation.
FIG. 1 shows the bandgap of such a GaAs—GaN mixed crystal system together with other group III-V compound semiconductor materials (Kondow, M., et al., Extended Abstracts of the 1995 International Conference on Solid State Devices and Materials, Osaka, 1995, pp. 1016-1018).
Referring to FIG. 1, it should be noted that, while GaN or a mixed crystal thereof containing a small amount of As has a very large bandgap suitable for emission of blue or violet optical radiation, the mixed crystal of GaAs containing a small amount of N has a small bandgap suitable for emission of the 1.3 μm or 1.5 μm optical wavelength band used for optical telecommunications systems. It should be noted that the bandgap of the GaAs mixed crystal decreases rapidly with increasing N content therein. Further, FIG. 1 indicates that the lattice constant of the GaAs mixed crystal decreases substantially with increasing N content therein.
Thus, the Japanese Laid-Open Patent Publication 6-334168 describes a technology of growing a III-V mixed crystal film containing N on a Si substrate epitaxially. For example, the foregoing reference describes a laser diode and a photodiode that use a GaNP cladding layer having a composition of GaN0.03P0.97 in combination with an active layer having a strained superlattice structure in which a GaNP layer and a GaNAs layer are stacked alternately and repeatedly. The foregoing cladding layer successfully establishes a lattice matching with the Si substrate. According to the teaching of the foregoing reference, it becomes possible to form a III-V device on a Si substrate without inducing misfit dislocations in the epitaxial layers. Further, the disclosed technology enables formation of an integrated circuit in which the III-V optical semiconductor devices are integrated monolithically with other Si devices.
Further, various mixed crystal compositions that establish a lattice matching with a substrate of GaAs, InP or GaP are reported for various N-containing III-V systems such as GaInNAs, AlGaNAs and GaNAs in the Japanese Laid-Open Patent Publications 6-037355.
Conventionally, no III-V composition has been known that has a bandgap smaller than the bandgap of GaAs and simultaneously a lattice constant that matches the lattice constant of GaAs, until a mixed crystal of GaInNAs is discovered. Provided that the N content is held small, the GaInNAs mixed crystal successfully establishes a lattice matching with a GaAs substrate and simultaneously has a bandgap smaller than the bandgap of GaAs. See the band diagram of FIG. 1. Thus, the GaInNAs mixed crystal is thought to be a promising material for an active layer of an optical semiconductor device that operates in the 1.3 μm or 1.5 μm wavelength band. However, little is known about the properties of the mixed crystal of GaInNAs.
Thus, Kondow, M., et al., op. cit., proposes a laser diode structure that uses a GaInNAs mixed crystal for the active layer of a laser diode. The reference further discloses the use of a cladding layer of AlGaAs in contact with the active layer of GaInNAs for securing a large discontinuity in the conduction band at the heterojunction interface across the cladding layer and the active layer. Because of the very large band discontinuity at the heterojunction interface, the laser diode is expected to show a high efficiency of laser oscillation and improved temperature characteristic associated with an efficient confinement of the carriers in the active layer.
On the other hand, It is known that the epitaxial growth of a GaInNAs mixed crystal is substantially difficult at high temperatures because of the tendency of the N atoms escaping from the deposited epitaxial layer of GaInNAs In order to obtain a film containing a substantial amount of N atoms, it is necessary to carry out the deposition process at a temperature of about 680° C. or less. However, the epitaxial growth at such a low temperature is not preferable for the growth of a layer containing Al, such as a layer of AlGaAs used for the cladding layer, because of the tendency of the highly reactive Al atoms in the cladding layer reacting with a small amount of O atoms remaining in the deposition chamber or in the source gases as impurity. The O atoms thus incorporated form a non-optical recombination center in the epitaxial layer, while the non-optical recombination centers thus formed tend to annihilate the carriers without emitting photons. It should be noted that the problem of oxidation of Al cannot be avoided even when the deposition is carried out under an environment where the air is purged by a high-performance vacuum system.
In order to avoid the foregoing problem of incorporation of the O atoms into the cladding layer of AlGaAs, it is necessary to carry out the deposition of the cladding layer at a high temperature of at least 750° C. However, the use of such a high temperature growth is contradictory to the requirement of low temperature growth of the GaInNAs active layer as noted previously. Even in the case in which the substrate temperature is lowered after the deposition of the cladding layer of AlGaAs for allowing the deposition of the GaInNAs layer thereon, such a lowering of the substrate temperature also allows unwanted incorporation of the O atoms in the reaction chamber to the exposed surface of the lower cladding layer, and the formation of non-optical recombination centers at the heteroepitaxial interface between the lower cladding layer and the active layer is inevitable. It should be noted that the non-optical recombination centers reduce the lifetime of the optical semiconductor device such as a laser diode.
In addition to the foregoing problem, the inventor of the present invention has discovered that a direct epitaxial growth of a GaInNAs layer on an AlGaAs layer is difficult, as in the case of forming a laser diode that uses a cladding layer of AlGaAs in combination with the GaInNAs active layer.
FIG. 2 shows the surface morphology of a GaInNAs layer grown directly on an epitaxial layer of AlGaAs, which in turn is grown on a GaAs substrate. The GaInNAs layer is grown with a thickness of 0.1 μm and has a composition of Ga0.9In0.1N0.03As0.97 for a successful lattice matching to the GaAs substrate. The AlGaAs layer underlying the GaInNAs layer has a composition of Al0.4Ga0.6As and is grown with a thickness of about 0.2 μm.
Referring to FIG. 2, it will be noted that the surface of the GaInNAs layer is not smooth but includes minute projections and depressions, indicating a non-uniform or island-like growth of the GaInNAs layer occurring on the surface of the AlGaAs layer. The GaInNAs layer having such an irregular surface morphology performs poorly when used for the active layer of a laser diode due to various reasons such as scattering of light at the irregular surface or non-optical recombination of the carriers caused by the defects that accompany with such irregular heteroepitaxial interface.
Thus, it has been difficult to fabricate a double hetero laser diode that uses a III-V layer containing N for the active layer.